Effect of Mechanical Vibration with Different Frequencies on Pore Structure and Fractal Characteristics in Lean Coal

Maoliang Shen,Xuexi Chen,Yong Xu,Mickaël Lallart

doi:10.1155/2021/5587592

Abstract

The mechanical vibrations caused by underground operations can easily lead to coal and gas outbursts in coal mines. Using the MVGAD-I experimental platform that we designed, the raw coal (0 Hz) was treated with vibration frequencies of 25, 50, 75, and 100 Hz, and the coal samples with different frequency vibrations were obtained. The total pore volume (TPV), specific surface area (SSA), pore size distribution, and the pore fractal dimension (PFD) of five coal samples were analyzed by mercury intrusion porosimetry and low-pressure nitrogen adsorption data. We found that the TPV, SSA, and PFD of the coal samples fluctuate with the increase of vibration frequency. The changes of the TPV and SSA of coal samples treated with 25 and 75 Hz vibrations were significantly greater than those subjected to vibrations of 50 and 100 Hz. Compared with the raw coal (0 Hz), the TPV and SSA of macropores, mesopores, and micropores increased the most in 75 Hz vibration coal sample. Therefore, the 75 Hz vibration excitation can improve the permeability of a body of coal mass and is conducive to the diffusion and seepage of coalbed methane and its production.. The influence of 25 Hz vibration on the TPV and SSA of macropores and mesopores is not obvious, but the TPV and SSA of minipores and micropores decrease significantly, which is not conducive to gas diffusion and adsorption. In addition, 25 and 75 Hz vibrations obviously damaged the fractal characteristics of both mesopores and micropores, resulting in the change of gas adsorption and diffusion ability. The rational use of a 75 Hz vibration is beneficial to both the production of gas and the prevention of outbursts, while a 25 Hz vibration should be avoided. The results are expected to reveal the microscopic mechanism of a vibration-induced outburst and provide theoretical guidance for employing the appropriate frequency of vibration to improve the rate of gas drainage and reduce the risk of outbursts.

Highlights

Coal and gas outbursts are one of the major disasters that threaten the safety of coal mine production [1,2,3]
Coal and gas outbursts occur in areas that have been disturbed by tectonism, and the coal bodies in those areas are characterized as crumpled and cracked [15]. e vibration caused by a “small disturbance” may lead to a change in the pore structure of the coal, which has an important influence on its permeability, coal strength, elastic modulus, gas adsorption, and diffusion [16,17,18,19,20]. erefore, it is of great significance to study how vibration affects the evolution of pores and fissures in the coal to reveal the microscopic mechanism of “small disturbances” that affect the instability of coal rock and the gas adsorption and diffusion characteristics of the coal seam
When the mercury injection pressure is below 0.1 MPa (D ≈10000 nm), mercury mainly penetrates the intergranular pores [52], and when the mercury injection pressure is greater than 10 MPa (D ≈120 nm), the coal matrix is compressed and this deformation causes pore damage [42]. erefore, mercury intrusion porosimetry (MIP) is mainly used to analyze the structural characteristics of mesopores and macropores

Summary

Introduction

Coal and gas outbursts are one of the major disasters that threaten the safety of coal mine production [1,2,3]. E vibration caused by a “small disturbance” may lead to a change in the pore structure of the coal, which has an important influence on its permeability, coal strength, elastic modulus, gas adsorption, and diffusion [16,17,18,19,20]. Erefore, it is of great significance to study how vibration affects the evolution of pores and fissures in the coal to reveal the microscopic mechanism of “small disturbances” that affect the instability of coal rock and the gas adsorption and diffusion characteristics of the coal seam. While affecting the adsorption and desorption capacity of coal to gas, the stress wave generated by the vibration can accelerate the development of internal cracks in the coal and rock mass, resulting in increased permeability of the coal rock [24,25,26], and the permeability increases exponentially with the increase of frequency [27]. The nan and microscale pores and fissures are directly related to the gas desorption and diffusion in the coal and the mechanical properties of coal [16, 33, 34]

Methods

Results

Conclusion